Vibration control devices can be used in a number of areas including machinery, transportation equipment, vehicles, scientific instruments, buildings, bridges, and elsewhere.
Vibration-control devices in civil engineering applications frequently utilize rubber bearings to support service loads, and also to change the natural period of civil structures to mitigate response from the natural hazards such as earthquakes. While the internal stresses of structures can be reduced with the installation of rubber bearings, severe deformation of girders can occur during large earthquakes, especially at near-fault locations. Therefore it can be desirable to also control the structural displacements that occur during an earthquake.
Control of such structural displacements is also desirable for structures utilizing continuous beam construction. Because of the advantages of less deformation and higher degree of driving comfort, continuous beam bridges are a commonly used structure form for highway and city bridges. However, in recent history, many highly destructive earthquakes have occurred in the world, among these earthquakes are the Northridge earthquake in the US in 1994, the Kobe earthquake in Japan in 1995, the Chi-chi earthquake in Taiwan in 1999, the Wen-Chuan earthquake in China in 2008, the Haiti earthquake and Chile earthquake in 2010 which all resulted in many continuous beam bridge deformations and collapses that blocked traffic and contributed to enormous death and economic loss. For example the BaiHua Bridge in Wen-Chuan earthquake, which was very close to the causative fault (the closest point was 1.5 km from the bridge), was damaged and the damage to this bridge added great difficulty to relief efforts and resulted in secondary problems. Further, because it is unlikely that the placement of bridges near to earthquake faults can be avoided, the potential for collapse of those bridges and resulting injury and economic loss will remain. Accordingly, improved anti-earthquake capabilities of continuous bridges and improved bearing designs for continuous beam structures is desirable.
One approach to reduce the damage to continuous beam bridge caused by earthquakes includes the use of damper and lead rubber bearings. Other approaches include intelligent materials, for example magnetorheological elastomer (MREs). MREs utilize both elastomeric and magnetorheological materials and can provide advantages over magnetorheological fluids, which can have problems such as sedimentation/settling of the magnetic particles, poor service life and leakage. MREs can be used in some applications for vibration control in fields such as aerospace and mechanics.
As a kind of intelligent material, a Magneto Rheological Elastomers (MRE) can be a solid or gel matrix with magnetic particles dispersed therein, and can reform upon application of a magnetic field into a solid and ordered structure whose modulus of elasticity and other mechanical properties are significantly increased, and this reformation can be quickly completed and can have good reversibility. MREs can have the advantages of both magnetorheological materials and of elastomers and can offer advantages over magnetorheological fluids of improved suspension of the magnetic particles, stability and ease of encapsulating the magnetorheological material. As such, MREs can be used in automotive applications, such as suspension systems, motor mounts and car bumpers as well as in artificial muscle applications. MREs can also be used in the fields of Aerospace and mechanical vibration control, bionics and elsewhere. However, traditional MREs, those without added carbon nanotubes, have limited usefulness due to the basic matrix being too soft.
Magnetorheological elastomers, including those with carbon nanotubes, because of their advantages of quick response, good reversibility and frequent usability in applications where rubber bearings can be used, also have potential use as a smart bearing to reduce the effect of an earthquake. In some embodiments, an MRE with carbon nanotubes can replace the rubber portion of a isolator to make a multilayer intelligent magnetorheological elastomer isolator. For some designs, finite element analysis shows that the magnetic flux density for some embodiments of an MRE with carbon nanotubes can be up to 1.2 T and that the magnetic field can activate the magnetocaloric effect of magnetorheological elastomers.
MRE functionality and capabilities can be improved and extended with the addition of carbon nanotubes. In various embodiments, the MRE with carbon nanotubes can exhibit improved shearing stiffness and capacity for consuming energy. In addition, these properties can be adjusted by an external magnetic field.
Current technological solutions associated with rubber bearings frequently include the installation of passive fluid viscous dampers with lead cores added to the bearings. However, these techniques cannot adjust the structural responses with either active or semi-active control. While one significant function of rubber bearings is to provide support against loads, the installation of passive fluid viscous dampers does not have sufficient supporting ability for many applications. Further, in some installations, more space is required to install this type of damper which can be difficult to accommodate in an engineering design. In addition, the production of rubber bearings with lead cores is complicated, and the resulting bearings frequently cannot achieve as good of results when compared with semi-active devices because of the intrinsic mechanisms of passive energy dissipation in these devices.
Magnetorheological (MR) nanocomposites can be reinforced with carbon nanotubes, such as single-walled and/or multi-walled carbon nanotubes and in various embodiments can result in nanocomposites which can exhibit higher zero-magnetic-field stiffness, improved damping performance or higher magnetic-field-induced increases in stiffness and dampening as well as combinations of more than one of these properties, such as improved stiffness and dampening and improved stiffness, dampening and magnetic-field-induced increases in stiffness and/or dampening as compared to MR materials without carbon nanotubes.
Magnetorheological (MR) elastomers can be smart materials whose viscoelastic properties can be controlled rapidly and reversibly by applied external magnetic fields. Microstructurally, MR elastomers can be composed of an elastomeric matrix, such as a low-permeability elastomeric matrix, and ferromagnetic particle fillers, and can have anisotropic or isotropic structures and/or properties. In one approach to fabricating anisotropic MR elastomers, magnetic fields can be applied during the curing process, which can cause particles to form chain-like structures. In one approach to fabricating isotropic MR elastomers, no magnetic field is applied during the curing process, particles can be more randomly dispersed in the matrix, thus showing a more isotropic structure. When cured MR elastomers are exposed to applied magnetic fields with direction parallel to the chain-structures (when present), the magnetic interaction among the particles can induce a prompt and continuous change in their stiffness and damping, which is usually referred to as the “MR effect.” In some cases, MR elastomers can operate in two modes: compression and shear. In some situations, only one of compression or shear will be present and in some situations a combination of compression and shear can be present. For some uses of anisotropic MR elastomers in compression mode, loads can be applied parallel to the chain-structures, and for some uses of anisotropic MR elastomers, such as in shear mode, the loads can be applied perpendicular to the chain-like structures.
Frequently, a prominent MR effect is desired. This desirability in some cases can reflect a wide range of adjustability of properties due to the MR effect. In various situations, the MR effect can include an absolute or a relative (as compared to the no-magnetic-field condition) changes in stiffness and/or damping, and in some situations, the MR effect can include both an absolute and a relative change in stiffness and/or damping. A simultaneously high absolute and relative MR effect can hardly be difficult to achieve because a high relative MR effect is more easily achieved with low initial (no-magnetic-field) values for MR-effected properties, and high absolute MR-effected properties are generally aided by high initial properties. Accordingly, when a certain level of initial properties is demanded, it can be advantageous to consider how to increase the absolute MR effect.
Carbon nanotubes (“CNT”) can be used to reinforce matrix polymers, such as those used for MREs, and can modify various properties including stiffness, strength and/or damping properties. In some cases, CNT-reinforced composites have shown dramatic improvement in stiffness, strength, and damping properties. In some situations, however, the strain at failure can be decreased. Without wishing to be bound by theory, it is believed that the small size and high aspect ratio of CNTs leads to a dramatic increase in surface area, hence a significant volume fraction of strong interfacial region, which can result in a significant enhancement in modulus and strength for the polymer nanocomposites, even when small amounts of nanofillers/carbon nanotubes are present. It is also believed that increased damping in CNT reinforced matrix polymers is due to energy dissipation due to interfacial debonding and sliding between CNTs and the polymer matrix. It is also believed that the interfaces between magnetic particles and matrix polymer strongly influence the MR effect of MR elastomers. As such, added materials, such as carbon black added to natural-rubber-based MR elastomers can result in microstructures with better bonding between carbon nanotubes and the polymer matrix, which in some cases is believed to lead to a higher MR effect but can also result in a lower absolute value for damping.
The magnetorheological elastomers with carbon nanotubes of the present disclosure, as compared to other types of dampers, such as passive viscous dampers, rubber bearings and/or lead-core rubber bearings, conventional magnetorheological elastomers and magnetorheological fluids (MRF), can have multiple advantages including:                1. Compared to the passive viscous dampers, the magnetorheological elastomers with carbon nanotubes require less space.        2. Compared to the conventional rubber bearing and/or lead-core rubber bearing, the magnetorheological elastomers with carbon nanotubes are semi-active (controllable and on-demand).        3. Compared to the conventional magnetorheological elastomers, the magnetorheological elastomers with carbon nanotubes have higher modulus and damping ratio including when multi-walled carbon-nanotubes are used as fillers.        4. Compared to the magnetorheological fluid, the novel magnetorheological nanocomposites not only have no particle settling problems, but also have more rapid responses to the control signals.        
In addition, bearings and/or dampers which utilize the magnetorheological elastomers with nanotubes disclosed herein can be designed to incorporate the functions a lock-up device and/or a shear-key.